Digestion - Stomach and Pancreas

Function and location of parietal cells

Secretion of HCl and intrinsic factor

Epithelial lining of stomach in gastric pits

Function and location of Enterochromaffin-like cell

Releases histamine

Gastric pit epithelium

Function and location of G cells

Produce gastrin

Gastric pit epithelia

Function and location of D cells

Produce somatostatin (which inhibits acid production)

Function and location of mucous neck cells

Secrete mucous and bicarbonate

gastric pit epithelium

3 molecules that stimulate gastric acid secretion

Gastrin

Histamine

ACh

Function and location of chief cells

Release pepsinogen - protein digesting enzymes

gastric pit epithelium

What is the endocrine signal that stimulate gastric acid production

Gastrin - is released by G-cells into blood, where it then reaches parietal cells

What is the paracrine signal that stimulate gastric acid production

Histamine - is released from a neighbouring gastric ECL (enterochromaffin-like) cell to the parietal cell

What is the neurocrine signal that stimulate gastric acid production

ACh - Is released from neurons of Vagus nerve to parietal cell

Most powerful acid secreting molecule

Histamine

Structural differences between a resting, and stimulated parietal cell

resting

• Full of tubulovesicles containing H⁺/K⁺ ATPase (proton pumps), which come together to increase surface area when required

Stimulated

• Tubulovesicles come together to form canaliculi to increase SA, and insert proton pumps on the surface

Why does blood pH increase when HCl is created

In the diagram is shows for every HCl made, it requires the movement of a HCO3 bicarbonate into the blood, thus raising the pH of the blood as it absorbs the circulating H+ ions in the blood. - this temporary rise in blood pH is called alkaline tide

3 phases of gastric secretion

1. Cephalic - brain

2. Gastric - stomach

3. Intestinal - intestines

What happens in the cephalic phase and what triggers it

Neural trigger - sight/smell/thought of food, chewing and swallowing can also trigger this

Releases ACh

What happens in the gastric phase and what triggers it

Mechanoreceptors detecting stomach stretch and chemoreceptors checking for peptides and amino acids in stomach.

Releases gastrin

What happens in the intestinal phase and what triggers it

Inhibitory phase - triggered by HCl in duodenum.

Releases somatostatin, secretin, and CCK, all inhibitors of HCl production in parietal cell.

Extra CCK release can be triggered by lipid presence

CCK also stimulates HCO₃^- release, neutralising HCl in duodenum

• Also stimulates digestive enzyme and bile release

Where do pancreatic juices and bile enter the duodenum?

Ampulla of Vater

Function of secretin

Stimulates bicarbonate release, to neutralise HCl, and inhibits HCl production via stimulating somatostatin release

What endocrine cells are in the pancreas and what do they produce?

Islets of Langerhands

produce somatostatin, glucagon, and insulin

What exocrine function does the pancreas perform?

Acinar cells release enzymatic juices into duodenum, which break down most digestible foodtypes

Duct cells secrete aqueous NAHCO₃ solution

How is pancreatic secretion regulated?

• Minor secretion during cephalic and gastric phases, as stimulation by vagal nerve fibres cause release of pancreatic juice

1. Acidic chyme causes secretin to release, and fatty/protein-rich chyme causes release of cholecystokinin (CCK). These molecules come from the enteroendocrine walls of the duodenum

2. CCK and secretin enter bloodstream

3. When CCK and secretin reach pancreas, they stimulate release of enzymatic pancreatic juice, and bicarbonate-rich juice respectively.

Functions of CCK

Stimulate gall bladder bile release

Stimulate enzyme-rich pancreas secretion

Stimulate HCO₃^- release

How does the duct cell in the pancreas neutralise alkaline tide

Duct cell create HCO₃^- molecules by breakdown of carbonic acid. It releases the bicarbonate into the lumen of the duct for release into duodenum, and releases H⁺ into the blood, neutralising the bicarbonate in the blood from acid production.

Why are pancreatic enzymes secreted as proenzymes (zymogens)

To prevent autodigestion of the pancreas.

When do the pancreatic zymogens become active enzymes, and what enzyme oes this

Trypsin causes the zymogens to become enzymes when they reach the small intestine